PATENT DOCUMENT 1 describes an exhaust valve device for a turbo charged engine. The exhaust valve device is provided between a turbine and an independent exhaust passage communicating with each of cylinders. Depending on an engine speed, the exhaust valve device changes a flow area of exhaust gas discharged from the engine to change a flow rate of the exhaust gas to be introduced into the turbine.
Further detailed below is the exhaust device described in PATENT DOCUMENT 1. This engine is an inline-four engine having four cylinders including first to fourth cylinders. The independent exhaust passage includes a first exhaust passage communicating with the first cylinder, a second exhaust passage communicating with the second and third cylinders, and a third exhaust passage communicating with the fourth cylinder. The exhaust valve device includes an upstream exhaust passage connected to the independent exhaust passage. A turbocharger includes a downstream exhaust passage connecting the upstream exhaust passage to a turbine housing.
The upstream exhaust passage includes three independent passages each communicating with a corresponding one of the first to third exhaust passages. Each of the three passages branches into two passages; namely, a high-speed passage and a low-speed passage. The downstream exhaust passage includes (i) independent low-speed passages communicating with respective low-speed passages of the upstream exhaust passage and (ii) independent high-speed passages communicating with respective high-speed passages of the upstream exhaust passage. Three independent low-speed passages of the upstream exhaust passage join the low-speed passages of the downstream exhaust passage, and three independent high-speed passages in the upstream exhaust passage join the high-speed passages of the downstream exhaust passage. The downstream exhaust passage has a downstream end at which the low-speed passages and the high-speed passages join together. The join of the downstream end is connected to an inlet of the turbine.
A high-speed passage of the upstream exhaust passage is provided with a butterfly valve. The butterfly valve switches between an open position and a closed position when a drive shaft connected to the butterfly valve is rotated by an actuator.
When the engine runs in a low speed range; that is, the engine runs at a predetermined speed or lower, the butterfly valve closes. The closed butterfly valve narrows down the flow area of the exhaust gas, raising the flow rate of the exhaust gas and increasing driving force of the turbine in the low speed range of the engine. Moreover, the increase in the flow rate of the exhaust gas creates an ejector effect at the join of the three independent passages for low speed passages. In the low speed range, the increase in the flow rate also creates an effect to suck burned gas out of a cylinder. Meanwhile, in the high speed range of the engine, the exhaust gas can be introduced into the turbine through both the low-speed passages and the high-speed passages, reducing exhaust backpressure and increasing driving force of the turbine.
PATENT DOCUMENT 2 describes that fuel is cut if a predetermined fuel cut condition is met while the car is slowing down, so that the fuel efficiency of the car improves.